MULTI-PHASE STEEL, COLD-ROLLED FLAT PRODUCT WHICH IS PRODUCED FROM A MULTI-PHASE STEEL OF THIS TYPE, AND METHOD FOR PRODUCING IT

摘要

Multi-phase steel comprises (in wt.%): carbon (0.14 - 0.25), manganese (1.7-2.5), silicon (0.2-0.7), chromium (less than 0.1), molybdenum (less than 0.05), niobium (0.02-0.06), sulfur (up to 0.01), phosphorus (up to 0.02), nitrogen (up to 0.01), optionally at least one of titanium (up to 0.1), boron (up to 0.002) or vanadium (up to 0.15), and remaining iron and unavoidable impurities. The microstructure of the steel comprises at least 10 vol-% of ferrite and at least 6 vol-% of residual austenite and the steel has a tensile strength of at least 950 MPa. Multi-phase steel comprises (in wt.%): carbon (0.14 - 0.25), manganese (1.7-2.5), silicon (0.2-0.7), chromium (less than 0.1), molybdenum (less than 0.05), niobium (0.02-0.06), sulfur (up to 0.01), phosphorus (up to 0.02), nitrogen (up to 0.01), optionally at least one of titanium (up to 0.1), boron (up to 0.002) or vanadium (up to 0.15), and remaining iron and unavoidable impurities. The microstructure of the steel comprises at least 10 vol-% of ferrite and at least 6 vol-% of residual austenite and the steel has a tensile strength of at least 950 MPa, a yield strength of at least 500 MPa, and an elongation of at least 15% measured in the transverse direction. Independent claims are also included for: (1) a cold-flat product produced from the multi-phase steel; and (2) producing the cold-flat product, comprising melting and casting the multi-phase steel to an intermediate product, hot rolling the precursor starting from an initial temperature of 1100-1300[deg] C ending at a final temperature of 820-950[deg] C to a hot strip, coiling the hot band to a coiling temperature of 400 -750[deg] C, optional annealing the hot bands for improving the cold rollability, cold rolling the hot bands to the cold-rolled product, after coiling at a cold rolling degree of 30-80%, continuously annealing the cold-rolled product at an annealing temperature of 750-900[deg] C, optionally acceleratedly cooling the continuously annealed cold-rolled product, and overaging the cold-rolled product at an aging temperature of 350-500[deg] C.